Wireless communications take place in an increasingly noisy environment. Multiple sources and receivers compete to transmit and receive intelligible signals, competing in an electromagnetic tragedy of the commons as each signal becomes noise for every other signal. Various means have been devised to overcome this problem; these can be found in a variety of sources, including patents and technological journals. Some are herewith summarily described as the background of the current invention.
Most wireless electromagnetic communication networks (WECNs) have a core hierarchy of Base Stations (BS). A newer BS has a multiplicity of sector antennae spatially separated in a known configuration, and communicates with a penumbral scatter of individual subscriber units (SU). If each BS communicates over a different frequency, then each SU must either have a tuned receiver for each station to which the subscriber tunes or, more commonly, a tunable receiver capable of reaching the range of frequencies encompassing those BSs to which it subscribes.
The inherently multipoint nature of WECNs, i.e., their multiple origination and destination nodes, has spawned a growing demand for methods and apparatus that will enable each particular WECN to exploit its particular partition of the spectrum and geography in constantly-changing and unpredictable economic and financial environments. Efficient use of a particular constraint set of frequencies, power, and hardware, is more in demand than ever, as the competitive field and available spectrum grows more and more crowded.
The prior art includes many schemes for maximizing signal clarity and minimizing interference between nodes in a complex, multipoint environment. These include differentiation by: (a) Frequency channels; (b) time slots; (c) code spreading; and (d) spatial separation.
First generation systems (e.g. AMPS, NORDIC) developed for cellular mobile radio systems (CMRS) provide frequency-division multiple access (FDMA) communication between a BS and multiple SUs, by allowing each SU to communicate with the BS on only one of several non-overlapping frequency channels covering the spectrum available to the system. This approach allows each SU to ‘tune out’ those frequencies that are not assigned, or not authorized, to send to it. Intercell interference is mitigated by further restricting frequency channels available to adjacent BS's in the network, to ensure that BS's and SU's reusing the same frequency channel are geographically removed from each other beyond interference.
Under FDMA, the total number of channels available at each BS is therefore a function of channel bandwidth employed by the system and/or economically usable at the SU. Hardware and regulatory limits on total spectrum available for such channels, and interference mitigation needs of a cellular WECN (cellular reuse factor), effectively constrain the divisibility of the spectrum and thus the geographical interacting complexity of current networks. (i.e. if the hardware requires a 200 kHz differentiation, and the WECN has 5 MHz of spectrum available, then at most 25 separate channels are available to the WECN.) Channelization for most 1G cellular WECNs is 25–30 kHz (30 kHz in US, 25 kHz most other places; for 2G cellular is 30 kHz (FDMA-TDMA) for IS-136, 200 kHz for (FDMA-TDMA) GSM, 1.25 MHz for (FDMA-CDMA) IS-95; 2.5G maintains GSM time-frequency layout; and proposed and now-instantiated channelization for 3G cellular WECNs is FDMA-TDMA-CDMA with 5 MHz, 10 MHz, and 20 MHz frequency channels.
Most so-called second generation CMRS and Personal Communication Services (PCS) (e.g. GSM and IS-136), and ‘2.5 generation’ mobility systems (e.g., EDGE), further divide each frequency channel into time slots allocated over time frames, to provide Time Division Multiple Access (TDMA) between a BS and SUs. (For example, if the hardware requires at least 1 ms of signal and the polling cycle is 10 ms, only 10 separate channels are available; the first from 0 to 1 ms, the second from 1 to 2 ms, and so on.) The combination of TDMA with FDMA nominally multiplies the number of channels available at a given BS for a given increase in hardware complexity. This increase hardware need comes from the fact that such an approach will require the WECN to employ a more complex modulation format, one that can support individual and combined FDMA-TDMA, e.g., FM (for FDMA AMPS) versus slotted root-Nyquist π/4-DQPSK (for IS-136 and EDGE) or GMSK (for GSM). This substantially increases hardware complexity, and thus cost, for each node of the WECN.
Some second generation mobility systems (e.g. IS95), and most third generation mobility systems, provide code division multiple access (CDMA) between a BS and multiple SUs (for example, IS-136 provides FDMA at 1.25 MHz), using different, fixed spreading codes for each link. The additional “degrees of freedom” (redundant time or frequency transmission) used by this or other spread spectrum modulation can (among other advantages) mitigate or even exploit channel distortion due to propagation between nodes over multiple paths, e.g., a direct and reflection path (FIG. 4), by allowing the communicator to operate in the presence of multipath frequency “nulls” or outages that may be significantly larger then the bandwidth of the pre-spread baseband signal (but less than the bandwidth of the spread signal)
Different spreading code techniques include direct-sequence spread spectrum (DSSS) and frequency hop multiple access (FMHA); for each implemented in a WECN, the hardware at each end of a link has to be able to manage the frequency and/or time modulation to encode and decode the signal correctly. Spreading codes can also be made adaptive, based on user, interference, and channel conditions. But each increase in the complexity of spread spectrum modulation and spreading code techniques useable by a WECN increases the complexity of the constituent parts thereof, for either every BS and SU can handle every technique implemented in the WECN, or the risk arises that a BS will not be able to communicate to a particular SU should they lack common coding.
Finally, individual communication nodes of a WECN may employ further spatial means to improve communications capability, e.g. to allow BS's to link with larger numbers of SU's. Spatial means include using at particular nodes multiple antennae with azimuthally separated mainlobe gain responses to communicate with SU's over multiple spatial sectors covering the a service area. These antennae can provide space division multiple access (SDMA) between multiple SU's communicating with the BS over the same frequency channel, time slot, or spreading code, or to provide reuse enhancement by decreasing range between BS's allowed to use the same time slot or frequency channel (thereby reducing reuse factor required by the communication system). A BS may communicate with an intended SU using a fixed antenna aimed at a well-defined, fixed-angle sectors (e.g. Sector 1 being between 0 and 60 degrees, Sector 2 between 60 and 120 degrees, and so forth), or using an adaptive or “smart” antenna that combines multiple antennae feeds to optimize spatial response on each frequency channel and time slot. The latter approach can further limit or reduce interference received at BS or SU nodes, by directing selective ‘nulls’ in the direction of SU's during BS operations. (FIG. 5). This is straightforward at the BS receiver, more difficult at the BS transmitter, unless if the system is time-division duplex (TDD) or otherwise single-frequency (e.g., simplex, as commonly employed in private mobile radio systems), or if the SU is based at “large” platforms such as planes, trains, or automobiles, or are used in other applications. This approach can provide additional benefits, by mitigating or even exploiting channel distortion due to propagation between nodes over multiple paths, e.g., a direct and reflection path. A further refinement that has been at least considered possible to adaptive SDMA signal management is the use of signal polarization, which can double degrees of freedom available to mitigate interference or multipath at BS or SU receivers, or to increase capacity available at individual links or nodes in the network. However, current implementations generally require antennae and transmissions with size or co-location requirements that are infeasible (measurable in meters) for high-mobility network units.
Various combinations of TDMA, CDMA, FDMA, and SDMA approaches have been envisioned or implemented for many other applications and services using a WECN, including private mobile radio (PMR) services; location/monitoring services (LMS) and Telematics services; fixed wireless access (FWA) services; wireless local, municipal, and wide area networks (LAN's, MAN's, and WAN's), and wireless backhaul networks.
The proliferation of protocols, the availability and feasibility of bandwidths, and the auctioning of licenses can only act as palliatives to this underlying apparent conflict of interests amongst different WECNs; none of these can by themselves solve the fundamental problem inherent in a shared, common, wireless communications environment.